1. Field of the Invention
The present invention relates to a process for producing a camshaft with cams. More particularly, the present invention relates to a process for producing a remelted and chilled camshaft (a surface remelted chilled layer camshaft) by melting a sliding cam surface by TIG arc and forming a chill layer by self-cooling.
2. Description of the Related Art
In a camshaft with cams fitted into an engine for an automobile and the like, an sliding cam surface of each of the cams must have a superior wear-resistance. Accordingly, the cam is subjected to a surface hardening heat-treatment in which the sliding cam surface portion is melted by a high density energy, such as a TIG arc, a laser beam, or an electron beam, and is rapidly cooled by self-cooling to form a chill hardened layer (for example, cf. Japanese Unexamined Patent Publication (Kokai) Nos. 59-23156, 60-234168, and 60-234169 filed by the present applicant). In a production process of a remelted and chilled camshaft using such surface hardening treatment in the prior art, a plurality (two) of tungsten torches are used and a plurality (two) of cams are simultaneously treated at one step station. In this case, since the distance between the tungsten torches is more than 80 mm, even if a plurality (two) of arcs are simultaneously generated, the problem of magnetic arc blow (the phenomenon of disturbance of the arcs by the influence of magnetic fields generated by electric currents) is not serious.
Improvements in the designs of engines have led to a shortening of the distance between cams of a camshaft, and accordingly, the distance between torches is shortened. Therefore, if two arcs are simultaneously generated, they mutually interact and a magnetic arc blow is caused. For example, from experiments made by the present inventors, it was found that when arcs are generated at a torch distance of 28 mm by continuous application of a direct current of 100 A, a deviation of about 0.7 mm is brought about by magnetic arc blow. In the case of a pulsed arc, which has a better arc stiffness than a continuous arc, the torch distance is 23 mm and thus the deviation by magnetic arc blow is about 0.7 mm. When such a magnetic arc blow occurs, the portion melted by TIG arc on the sliding cam surface is deviated from the predetermined position and, as is seen from the cam section shown in FIG. 1, the end face portion 3 of the remelted and chilled layer 2 of the cam 1 sags or an unhardened surface portion 4 appears, with the result that an appropriate hardened surface sliding on a mating member (for example, a rocker arm) cannot be obtained. Note, reference numeral 5 in FIG. 1 represents a hardened layer of a heat-affected zone.
For preventing magnetic arc blow, a method can be considered in which a shaft portion corresponding to the intermediate position between two tungsten torches is adopted as a place at which the camshaft is connected to the positive electrode of a welding power source. But, the construction of this shaft position and a connecting member attached thereto is difficult, and if the contact is degraded to even a slight degree, sparks are generated and the shaft portion and the connecting member are damaged. Furthermore, when another cam is treated after the treated cam, the connecting member must be connected to a fresh shaft portion, and this operation is troublesome.